1. Field of the Invention
The present invention relates to an improved planer used in woodworking.
2. Description of the Related Art
As widely known, planers are often used for planing by rectilineal cutting in the field of woodworking. A conventional planer typically includes two to four blades attached to a cutterhead of an appropriate width. The planer roughly finishes the surface of wood by cutting chips each having a shape corresponding to a trochoid locus drawn by a cutting edge of each blade.
Up-milling, wherein each blade is rotated in the direction opposite to the feeding direction of wood, has advantages over down-milling, wherein each blade is rotated in the same direction as feeding of wood, and up-milling is thereby applied to most cases of planing with a planer. Such advantages include: shallower and flatter knife marks, little damage to the cutting edge of each blade, and easier disposal of chips.
In the up-milling process, however, the conventional planar generally causes some defects as torn grain or chipped grain, wooly grain or fuzzy grain, and raised grain. Especially, torn grain or chipped grain, which means a wood surface having one or plural cuts deeper than a desirable finished surface, badly affects the subsequent steps of woodworking, and hence effective prevention of torn grain is highly desirable.
Such torn grain may be attributable partly to blades, that is, abrasion or inadequate setting of the cutting edge of each blade, and partly to wood, that is, direction and strength of wood fibers or cross grain. In any case, a cutting force of each blade applied onto a portion of wood shallower than a desired finished surface causes adverse effects on a place of wood deeper than the desirable finished surface to destroy the wood fibers in the deeper portion.
A variety of measures have been taken to prevent such torn grain or chipped grain. One typical example of these measures includes giving an appropriate bevel to a rake face of a blade (a face on which chips slide) in order to make a cutting angle greater than a tooth angle, thereby reducing a component of a cutting force in a direction separating from a surface of the wood to eliminate or reduce effects of the cutting force on a wood portion deeper than a desirable finished surface. Another example of such measures includes mounting a chip breaker on the side of the rake face of a blade or alternatively shaping a part of the cutterhead located on the side of the rake face of a blade as a chip breaker, so as to turn and fold chips and thereby prevent undesirable fore split. Here the fore split means the fracture of wood fibers generated before the cutting edge of each blade, and at least part of such fore split appears as torn grain on the surface of the wood.
Although generally increasing the cutting resistance, these measures reduce the amount of torn grain as long as various cutting conditions, such as the type of wood, direction of fibers, degree of dryness, and undeformed chip thickness of each blade are set within specific ranges. All such measures are, however, directed to portions of the wood located after the cutting edge of each blade, for reducing torn grain generated before the cutting edge of the blade. These indirect methods can not sufficiently prevent torn grain when the cutting conditions are not in the specific ranges. Even when the bevel or chip breaker is finely set according changes in the cutting conditions, these measures are of limited effectiveness.